A method of continuing to track an object in a case where the object overlaps and is hidden behind another object has been known (Japanese Patent Application Publication No. 2007-334631). According to Japanese Patent Application Publication No. 2007-334631, it is determined whether the hidden object reappears based on estimated information about the object, and an identical identification number is assigned to the object when the object reappears.
In a case, however, where multiple objects are simultaneously hidden behind a different object and conditions of the multiple objects change while the multiple objects are being hidden behind the different object, the technique of the Japanese Patent Application Publication No. 2007-334631 is likely to make the estimated information deviate from the actual state and accordingly assign wrong identification numbers to the objects which reappear.